Radio-frequency power amplifiers used in resonant operation may be amplifiers of the classes D, E, F, or comparable amplifiers and can include switched current supplies that are operated in a resonant operation. These switched current supplies or radio-frequency power amplifiers used in resonant operation are referred to simply as “amplifiers” herein.
Alternating voltage energy can be used in plasma processes, gas discharge processes for thin film coating or etching, or for laser excitation (i.e., with loads that can have strongly-varying load resistance). The use of the above-mentioned amplifiers for such applications is generally known.
The basic principle of such an amplifier is to use at least one switching element that is switched on and off with a basic frequency to guide power in a clocked manner from a uniform power source via a circuit to a resonant circuit to the output of which the load is connected. Ideally, this resonant circuit ensures that only the basic frequency is passed on to the load and that the switching element(s) can be operated with minimum loss. The output power is controlled by the power supply at the uniform power source (e.g., a DC voltage source). A circuit between the power source and the switching element ensures that the current cannot change during one period of the basic frequency. One or more MOSFETs are frequently used for the switching element.
There are different designs of amplifiers of this type, which can be known as amplifiers of classes D, E, and F. Some amplifiers are adjusted to different applications or also slightly modified. An amplifier of this type ideally has no lossy components and is operated with high efficiency. Real amplifiers show small losses in the resonant circuit and mainly in the switch of the circuit. Amplifiers operating at output powers of up to some 10 kW still can achieve an efficiency of 90% and more.
Generally, a precondition for proper functioning of such amplifiers is that the load to which the amplifier is connected has a finite load resistance. The associated load may be a simple load, and can be a plasma (e.g., used in plasma processing) or a laser excitation path, but may also be a combination of such a load and an upstream matching network or a transfer line or a combination of both or even further parts.
The load resistance in a gas discharge process or in a similar processes is very high when the gas discharge has not been ignited, and the amplifier, therefore, is operated practically in an open circuit condition. Furthermore, such processes can generate arcs that represent a very low-ohmic load resistance (e.g., similar to a short-circuit). Moreover, such processes must cover a very large power range, often from only a few 10 W up to several 10 kW.
Experiments have shown that in many cases during such processes there is no constant alternating voltage at the basic frequency in the unloaded state or during a short-circuit of an amplifier, but rather that the basic frequency (e.g., 10–100 MHz, or, more particularly, 13 MHz) is applied at the output with a superposed low-frequency amplitude modulation. For example, the frequency of the low-frequency signal of a 3 kW amplifier operated without a load at the output of the amplifier can be between 200 kHz and 2 MHz and can have a voltage amplitude that is 100% of the ac output voltage amplitude of the basic frequency. A similar behavior can occur at low powers, for example for a 80 W amplifier at the output and correct terminal impedance. This behavior is undesired in such applications.
It is therefore desirable to reduce undesired subfrequencies at extreme operating states of the amplifier (i.e., during open- or short-circuited operation of the amplifier) and any intermediate operating state of the amplifier, and at different output powers from the amplifier. The efficiency of the amplifier should thereby not be reduced and the expense for stabilization measures should be comparatively small.